An ultra-thin polymer electrolyte for 4.5 V high voltage LiCoO2 quasi-solid-state battery

To achieve safe and high energy density solid-state batteries (SSBs), the weight of solid-state electrolyte (SSE) should be minimized and a high voltage and high specific capacity cathode should be used. Polyethylene oxide (PEO)-based polymer electrolytes (PEs) has been identified as the optimal SSE for SSBs owing to their versatile advantages. However, fabricating ultra-thin and high voltage stable PEO-based PEs is still challenging. Herein, an ultra-thin (8.1 µm) blending polymer electrolyte (BPE) is designed through blending PEO, Polymethyl methacrylate (PMMA) and Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP), and complexing with Succinonitrile (SN), Fluoroethylene carbonate (FEC) and LiTFSI plasticizers. Based on this design, Li|BPE|LiFePO4 quasi-solid-state battery (QSSB) can operate for 250 cycles with a capacity retention of 92.8%, 4.5 V high voltage Li|BPE|LiCoO2 QSSB exhibits 87% capacity retention after 80 cycles, with an average Coulombic efficiency ≥99.8%, which is much superior than 4.5 V Li|PEO|LiCoO2 SSB, whose capacity rapidly decays to 0 mAh/g after a dezen of cycles. DFT calculation suggests that blending PEO, PMMA and PVDF-HFP increase the electrochemical oxidation tolerance. Interface study by TEM and XPS discloses PEO-LiTFSI/LiCoO2 interface is unstable with a big amount of decomposed products from PEO-LiTFSI and from LiCoO2, while BPE/LiCoO2 interface is more stable without obvious decomposition, indicating the promising application of BPE for high energy density QSSBs.